Rotary abrasive device

ABSTRACT

An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot has a peripheral opening of less width than the major diameter of the slot, the unit embodying a retainer shaped and dimensioned to be retained in the slot against radial displacement and a web of a thickness enabling it to extend radially outward through the slot, the web and preferably also the enlargement being formed of flexible material, and a plruaity of flexible abrasive leaves anchored to the web of the retainer. In one form the entire retainer is made integral of a plastic such as nylon. In another the enlargement is primarily a rod or filler enclosed by sheet material and the adjacent flat sections of the sheet material provide the web.

United States Patent [1 1 Block July 24, 1973 [5 ROTARY ABRASIVE DEVICE 3,645,049 2/1972 Freerks et al 51/334 [75] Inventor: Aleck Block, Los Angeles, Calif. P E O n M S rima xaminert e im son [73] Assign ee: Merit Abrasive Products, Inc. Atmmg, Ensworth Roston p [22] Filed: May 4, 1971 21 Appl. No.: 140,243 1 ABSTRACT Related Application Data An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot has a peripheral [62] 2 21 3 13 I969 opening of less width than the major diameter of the slot, the unit embodying a retainer shaped and dimensioned to be retained in the slot against radial displaceg ment and a web of a thickness enabling it to extend ra- 58 Field oi".;;;;i;1."IIIIjI'I 511332-337 dially the SM, web and 5 179 182 also the enlargement being formed of flexible material, and a plruaity of flexible abrasive leaves anchored to [56] References Cited the web of the retainer. In one form the entire retainer is made integral of a plastic such as nylon. In another UNITED STATES PATENTS the enlargement is primarily a rod or filler enclosed by 2,871,632 2/1959 Cosmos 51/337 sheet material and the adjacent flat sections of the "5 3: sheet material provide the web. 0c

18 Claims, 8 Drawing Figures PAIENIED 3.747. 2835 saw 1 or 2 ROTARY ABRASIVE DEVICE This is a division of application Ser. No. 794,243, filed Jan. 27, 1969, now U.s. Pat. No. 3,648,417.

BACKGROUND OF THE INVENTION The invention relates to a flap-type rotary abrasive device in which a rotary hub structure is provided with longitudinal peripheral recesses of the general crosssectional configuration of a keyhole for the purpose of anchoring a circumferential series of abrasive units in the form of packs of flexible abrasive leaves. The term keyhole configuration refers to any type of peripheral recess which has a peripheral opening or entrance that is of a width dimension substantially narrower than the maximum width dimension of the recess proper. The corresponding abrasive units have integral anchoring portions of corresponding keyhole cross-sectional configuration, each anchoring portion having an enlargement captivated by the recess and having a thinner portion extending radially outward through the narrowed entrance of the peripheral recess.

In one general type of prior art construction exemplified by Block U.S. Pat. No. 3,141,269 and Gillett U.S. Pat. No. 3,212,219, each abrasive unit comprises simply a pack of abrasive leaves unified by suitable cement and shaped with an anchoring portion of the required keyhole configuration. In a second general type of prior art construction, each abrasive unit incorporates a retainer member of keyhole configuration having an enlargement confined by the recess and having a web extending radially outward through the narrowed recess entrance with flexible abrasive leaves stapled or otherwise attached to opposite sides of the outwardly extending web. In each instance the peripheral entrance or opening to a recess is of substantial radial extent and the web of the retainer is of a thickness for a snug fit in the entrances with the consequence that the retainer is rigidly fixed relative to the hub structure. This second general type of construction is exemplified by Block U.S. Pat. No. 3,058,269, and two pending patent applications by Russell W. Burns, namely application Ser. No. 706,461, filed Feb. 19, 1968 entitled ABRASIVE DRUM CONSTRUCTION and application Ser. No. 728,946, filed May 14, 1968 entitled DRUM WITH I-IELICAL ABRASIVE ELEMENT. The Bernstein et al. U.S. Pat. Nos. RE 25,025 and RE 25,026 are also pertinent. i

It has been found that structural failure, or at least shortened service life, of such abrasive units may be caused by repeated flexure of the abrasive units in the operation of the rotary device. Flexure of an abrasive unit is caused by acceleration every time an operating period is initiated-and, of course, flexure of an abrasive unit by impactagainst a workpiece can occur hundreds of times a minute.

In the first described prior art construction where a pack of leaves is formed with an integral anchoring portion, the whole pack may be repeatedly flexed relatively sharply near the periphery of the hub structure and such flexure may be destructive in two ways: first, by abrasive action among the leaves: and, second, by cracking of the binder that retains the abrasive particles on the sheet material of the leaves.

In the second described prior art construction where the abrasive leaves are mounted on a retainer and lie outside of the circumference of the rotary hub structure, the retainer is stressed in flexure and there is always the possibility that the retainer may be caused to fail by fatigue. Another problem arises in that the retainer is substantially more rigid than the packs of leaves mounted thereon and the retainer is necessarily of limited radial extent to terminate far short of the outer ends of the abrasive leaves. Thus the retainer reenforces the leaves only in an inner radial region and the leaves may flex sharply where the retainer terminates. In effect, the retainer serves to shift the flexure region of the leaves outward from the periphery of the hub structure.

There is a certain disadvantage in shifting the bending zone of the leaves radially outward in that the angle to which an abrasive leaf is bent by impact against a workpiece may be relatively small if the bending occurs near the inner end of the leaf but may be relatively large if the stiffening retainer shifts the bending action towards the outer end of the leaf.

The object of the present invention is to solve these problems, i.e., to minimize flexural stressing of the abrasive units on a rotary abrasive device of the character described.

SUMMARY OF THE INVENTION The objective of the invention is attained by mounting the abrasive units on the hub structure in a pivoted or hinged manner to give the abrasive units freedom to seek equilibrium positions relative to the forces that tend to affect the angle of the abrasive units relative to the rotary hub structure on which they are mounted. Angular acceleration forces tend to swing the abrasive units backwards and angular deceleration forces tend to swing the abrasive units forward. Impact forces caused by a workpiece in the path of the abrasive packs deflect the abrasive packs rearwardly. Centrifugal force tends to straighten up the abrasive packs to radial positions whenever the leaves are affected by inertia forces or by impact forces and thus cooperate with these latter forces to repeatedly flex the leaves hundreds of times each minute.

With the abrasive units free to swing through a substantial range of angles, however, the abrasive units simply follow and conform to the resultants of the forces to which they are subjected. With the leaves mounted on a hinged retainer, the retainer repeatedly changes its angular position to avoid subjecting the abrasive unit to any significant bending stresses when the rotary hub is either abruptly accelerated or abruptly decelerated. When the outer end of a pack of leaves makes impact against a workpiece that is being processed, the pack is deflected rearwardly but with the retainer of the pack free to swing backward, at least a portion of the angular deflection may be accomplished by hinge action of the retainer instead of by flexure of the pack and when centrifugal force tends to restore the deflected pack to radial position, hinge action on the part of the retainer may accommodate at least a portion of the restoration. I

The desired freedom for angular movement of the abrasive units relative to the rotary hub structure is provided by making the enlargements of the retainers rotatable in the captivating keyhole recesses and by making the peripheral openings or entrances to the keyhole recesses relatively wide and making the webs of the retainers relatively thin. The maximum possible range of relative angular movement of such an abrasive unit is determined by two opposite limit positions where the opposite sides of the recess entrance serve as stops and if an acceleration force or an impact force were to tend to swing the retainer beyond one of these stop positions, the web of the retainer would be locally stressed in flexure. In this regard a feature of the preferred practice of the invention is the concept of providing a base or heel portion of the abrasive unit to abut the cylindrical surface comprising the periphery of the hub structure to serve as means to stop the swinging movement of the abrasive unit before the web of the retainer makes damaging impact against a side surface of the recess entrance. As will be explained, the stopping of the swinging movement in this manner harm lessly stresses the web of the retainer in tension instead of undersirably stressing the web in flexure.

Although the base or heel stops the swinging movement of the abrasive unit without destructive flexure of the web of the retainer in the region of the entrance of the corresponding recess, nevertheless, impact forces as well as acceleration forces may tend to swing the abrasive unit to an even greater angular departure from the radial position of the abrasive unit with consequent relatively sharp bending of the abrasive leaves adjacent the outer end of the retainer. In the preferred embodiment of the invention the flexural stressing of the pack of leaves in this manner is reduced by employing a retainer that has a resiliently flexible web because the outer portion of such a flexible web bends at a radius that is too large for any significant destructive effect on the web. This yielding action of the outer end portion of the web reduces the severity with which the leaves are flexed just beyond the outer end of the retainer.

A further feature of the preferred practice of the invention is the concept of employing a retainer in a keyhole slot that is somewhat oversized relative to the cross section of the enlarged root of the retainer. An important advantage of this concept is that liberal tolerances may be permitted in the dimensioning of the enlargement of the retainer. A second feature of the invention that is related to this last mentioned feature is the concept of forming the retainer enlargement with curved surfaces to make line contact with the walls of the hub recess on opposite sides of the web of the retainer. With the oversized hub recess of either generally circular cross-sectional configuration or generally triangular cross-sectional configuration line contact is provided to minimize frictional resistance to the hinge action of the retainer and this line contact is provided even though a retainer enlargement is varied in diameter. In the preferred practice of the invention resistance to the hinge action of an abrasive unit is further minimized by making the retainer enlargement of a plastic that has a low coefficient of friction relative to the metal surfaces of the slot that captivates the enlargement.

The features and advantages of the invention may be understood from the following detailed dscription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a side elevational view of a selected embodiment of the invention with a portion of the structure broken away;

FIG. 2 is a greatly enlarged portion of FIG. 1 showing how two packs of abrasive leaves are mounted on opposite sides of the web of a flexible retainer that is free to swing relative to the hub structure on which the abrasive unit is mounted;

FIG. 3 is a face view of the abrasive unit shown in FIG. 2;

FIG. 4 is a view similar to FIG. 2 showing how a heel of the abrasive unit abuts the periphery of the hub structure to limit the rearward swinging movement of the abrasive unit when the hub structure is accelerated;

FIG. 5 is a similar view showing how under certain conditions the hingedly mounted abrasive unit may seek an intermediate equilibrium position;

FIG. 6 is an elevational view of an abrasive unit in which a pack of abrasive leaves is mounted on only one side of the web of the retainer;

FIG. 7 is a view similar to FIG. 2 showing how a retainer may comprise sheet material enclosing a core of solid material; and

FIG. 8 is a view similar to FIG. 2 showing how the peripheral recess in which an abrasive unit is hingedly mounted may be of generally triangular cross-sectional configuration instead of generally circular crosssectional configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 embodying the presently preferred practice of the invention indicates the construction of a flap-type abrasive wheel which comprises a hub structure, generally designated 14, and a peripheral array of replaceable abrasive units 12. The hub structure comprises a generally cylindrical body which is retained on a power-actuated shaft 15 by a nut 16 and washer 18. The hub body is formed with a circumferential series of peripheral recesses or slots 20 which may be described as of a keyhole cross-sectional configuration in that each slot has a peripheral opening or entrance 22 that is narrower in width than the maximum width dimension of the slot. In this instance, the enlarged portion of each slot is of generally circular configuration.

Broadly described, each of the abrasive units 12 includes what may be termed a retainer generally designated 25 having an enlarged portion 26 in the form of a longitudinal rod portion at the base of radial web 28, by means of which the retainer is rotatably captivated by the corresponding slot 20. The retainer serves as means to mount the abrasive unit 12 on the hub 14 in a pivotal or hinged manner to permit the abrasive unit to swing through a liberal range of angles relative to the hub body. For this purpose the enlargement 26 of the retainer 25 has curved concentric surfaces in contact with the walls of the slot and the retainer has a relatively thin radial web 28, the thickness dimension of which is substantially less than the width of the entrance 22 of the slot.

The retainer 25 may be of any suitable construction and may be made of any suitable material but it is contemplated that the radial web 28 will be resilient to a desirable degree to permit the web to be flexed slightly or bent. Thus the retainer 25 may be made of a suitable plastic such as nylon, which has a low coefficient of friction relative to the overhanging side walls of a slot 20.

The abrasive unit is completed by a plurality of abrasive leaves that are suitably attached to the web 28 of the retainer. As best shown in FIG. 2, the abrasive leaves 30 may be formed in two packs 32 which straddle the web 28 and are suitably attached thereto, for example, by means of staples 34.

I'Iingedly mounting each of the abrasive units 12 on the hub body 14 in the described hinged manner permits the individual abrasive units to seek equilibrium positions with respect to forces acting on the abrasive units, i.e. permits the abrasive units to conform to the instant resultant of such forces. Thus FIG. 2 shows how anabrasive unit 12 assumes a radial position if a predominate force acting on the abrasive unit is centrifugal force.

When the hub body 14 is initially accelerated to operating speed, the abrasive unit is swung rearwardly with respect to the direction of rotation of the hub body. A feature of the preferred practice of the invention is that the rearward limit position of the abrasive unit is determined by abutment of a heel or base portion 35 of the abrasive unit against the periphery of the hub body, it being contemplated that the heel portion 35 will stop the rearward inclination of the abrasive unit before the radial web 28 of the retainer makes contact with a side surface of the slot entrance 22. Thus the heel portion 35 serves as stop means to prevent destructive bending of the web 28 by a side surface of the slot entrance.

In FIG. 4 the arrow directed rightward from the center of gravity 36 of the abrasive unit represents inertia or acceleration force that urges the abrasive unit rearward. In reaction to this acceleration force the heel portion 35 of the abrasive unit presses against the periphery of the hub body and the arrow p indicates this force. The resultant of these two forces is indicated by the arrow t which resultant force places the web 28 under tension. Since the center of gravity 36 is a substantial radial distance from the retainer enlargement 26 that anchors the abrasive unit, the web 28 is slightly flexed in FIG. 3 but it is to be noted that the radius of the flexure of the web is so large that the bending stress to which the web is subjected is distributed and is of insignificant magnitude.

As the hub body 14 is accelerated centrifugal force is developed and soon dominates the acceleration force. Thus FIG. 5 shows an intermediate position of the abrasive unit which may be termed a transition position since it occurs as the Hub body approaches full acceleration. As indicated by the arrows in FIG. 5, the two forces acting on the abrasive unit 12 in the absence of a workpiece are acceleration force indicated by the arrow a and centrifugal force indicated by the arrow 0. The resultant of these two forces is the force t which is aligned with the retainer web 28 and places the retainer web under tension.

Whena workpiece is placed in the orbital path of the abrasive units 12, the repeated impacts of the abrasive unitragainst the workpiece deflect the packs 32 and thus cause rearward deflection of the packs. If the abrasive units were fixedly mounted on the hub body 14, the rearward deflection would be accommodated solely by flexure of the packs 32. An important advantage of hingedly mounting the abrasive units on the hub body,

however, is that a portion of the rearward deflection of an abrasive unit may be accommodated by momentary rearward hinge action of the abrasive unit with consequent lessening of the degree to which the abrasive unit itself is flexed. Thus the invention minimizes flexure of the abrasive unit and avoids any flexure of the abrasive unit that is of destructive or even deleterious magnitude.

A further feature of the invention resides in the fact that it is contemplated that the replaceable abrasive units 12 shown in FIG. 1 will be sufficiently close together to cause the abrasive units that are approaching a workpiece to reinforce the abrasive unit that is at the instant in contact with the workpiece. This feature may be understood when it is considered that impact of an abrasive unit 12 against a workpiece deflects the abrasive unit rearwardly with respect to the direction of rotation of the abrasive wheel and such rearward deflection is against an approaching abrasive unit that is being acted upon by centrifugal force. Thus the normal tendency of abrasive units to take strictly radial positions under the influence of centrifugal force is useful in causing approaching abrasive units to oppose the rearward deflection of whichever abrasive unit is actually processing the workpiece.

Another feature of the invention resides in the fact that centrifugal force is relied upon to maintain an abrasive unit normally in a radial position instead of reliance being placed on mechanical cooperation between the hub structure and the abrasive unit. In the above-mentioned Berstein Reissue patents, for example, an abrasive unit is held in radial position by the two heel portions of the abrasive pack snugly abutting the periphery of the abrasive wheel and such snug contact requiresclose dimensioning of the root portion of the retainer of the abrasive unit that extends into the peripheral structure of the abrasive wheel. If such an abrasive unit is designed to be moved endwise into its installed position as in the construction in the present disclosure, any necessity for the heel portions of the abrasive unit to make snug contact with the periphery of the abrasive wheel results in such a close fit as to make it difficult to slide the retainer of the abrasive unit endwise into engagement with the keyhole slot of the abrasive wheel. Thus if the dimension of the retainer of the abrasive unit as measured radially of the abrasive wheel is too short, the retainer may jam when an attempt is made to slide the retainer endwise into the keyhole slot. On the other hand, if the dimension of the retainer of the abrasive unit measured radially of the abrasive wheel is too large, the heel portions of the abrasive pack will not make the desired snug contact with the periphery of the abrasive wheel to prevent shearing stress on the web of the retainer at the peripheral edge of the restraining slot.

In the present disclosure, centrifugal force is depended upon to position an abrasive unit radially of the abrasive wheel and cooperation between the heel of an abrasive unit and the periphery of the abrasive wheel is used merely 'to limit the degree to whichan abrasive unit leans rearward with respect to the direction of rotation of the abrasive wheel. Thus the retainers of the abrasive units in the new construction may be liberally dimensioned radially of the abrasive wheel to facilitate ready insertion of the retainers endwise into the keyhole slots of the abrasive wheel.

FIG. 6 shows how an abrasive unit 12a may comprise a single pack of abrasive leaves attached to one face of the retainer web 2%, the pack being on the side of the web for the heel portion 35a of the pack to serve as a stop when the abrasive unit is inclined rearward by acceleration force.

It is to be understood that the retainer 25 may be of various constructions within the scope of the invention. For example, FIG. 7 shows an abrasive unit 12b which is similar to the previously described abrasive unit except for the construction of the retainer 25b. The retainer 25b comprises a suitable fabric sheet 38 which encases a solid cylindrical core 40 and may be adhesively bonded thereto. The two end portions of the fabric sheet 38 may be bonded together and they fonn a double layer radial web 28b. The core 40 may be a short piece of metal rod or may be a piece of extruded plastic rod or may be a mass of suitable plastic material that is molded in contact with the sheet material.

As an alternative, the core 40 may be disposed between the two layers of the fabric sheet 38 without being bonded to the layers. As a further alternative, the two layers of the fabric may be disposed in contiguous relationship and the enlarged portion corresponding to the portion 26 in FIGS. 2 to 5, inclusive, may be formed by adhering suitable material such as nylon or an epoxy to one or both of the fabric layers. The fabric sheet 38 may be made from a suitable material such as nylon sheet material commonly used for automobile seats.

As previously mentioned, the entire retainer may be made of a plastic such as nylon, formed as an integral member as illustrated in FIG. 2 and in other figures. The benefits are numerous. Nylon is in a sense self lubricating, that is it slides readily for endwise ineration in a slot, and it rotates readily against the wall of the slot. It will not crystallize and fatigue as some metals do. It is easier on the drum than some metals. The web portion, through which the staples are driven, presents a somewhat resilient embedment for the staples, tending to less breakage of the staples under stress and impact of the abrasive unit against the workpiece.

Some of the foregoing advantages realized in a retainer in which the enlarged portion and the web are integrally formed of nylon or some other plastic having comparable characteristics may also be obtained in the two part retainer which is formed of sheet material encasing a core.

FIG. 8 indicates how an abrasive unit 12 may be mounted in a peripheral slot of the hub structure that differs in cross section from the slot 20 in FIGS. 1 3. The peripheral slot 20 of the hub body 1411 in FIG. 8 is of generally triangular cross-sectional configuration with a narrowed entrance 22. Both the hub slot 20a of FIG. 8 and the previously described hub slot 20 of FIG. 2 are what may be termed oversized relative to the cross-sectional dimension of the retainer enlargement 26. With a slot liberally dimensioned in this manner relative to the retainer enlargement, wide tolerance in the dimensioning of the retainer enlargement is permitted and a retainer enalrgement of relatively small cross dimension may be substituted for a retainer enlargement of relatively large dimension without change in the mode of operation of the abrasive unit. It is to be noted that both in FIG. 2 and in FIG. 8 when centrifugal force is effective the curved surfaces of the retainer enlargement makes line contact with the peripheral slot on opposite sides of the retainer web 28, the retainer enlargement making substantial tangential contact with each of the two overhanging side walls of the slot along a line that is parallel with the axis of the retainer enlargement.

My description in specific detail of the selected embodiments of the invention will suggest various changes, substitutions and other departures from my disclosure.

I claim:

1. An abrasive unit for pivotally mounting in a transverse peripheral slot of a rotary hub structure wherein the recess is'of the general cross-sectional configuration of a keyhole with a peripheral entrance of substantially less width than the maximum width dimension of the slot, comprising:

a retainer having an enlargement shaped and dimensioned to rotate in captivity in the slot and having a web to extend through the entrance of the slot, said web being of substantially less thickness than the width of said entrance of the slot to permit rotation of the enlargementin the slot; and

a plurality of flexible abrasive leaves anchored to said web of the retainer.

2. An abrasive unit as set forth in claim 1 in which the thickness of the web is dimensioned relative to the width of said entrance for a given range of angular movement of the retainer between opposite limit positions determined by the opposite surfaces of the entrance to the slot; and

in which the plurality of abrasive leaves is positioned and dimensioned to form a heel for abutment with the periphery of the hub structure before the retainer reaches a limit position thereby to prevent the web from being forced against a side surface of the entrance to the slot.

3. An abrasive unit as set forth in claim 1 in which the web is resiliently flexible.

4. An abrasive unit as set forth in claim 1 in which the retainer is made of plastic material having a relatively low coefficient of friction relative to the surfaces of the slot.

5. An abrasive unit as set forth in claim 1 in which the retainer comprises sheet material encasing a solid core to form the retainer enlargement and in which two layers of the sheet material form the web of the retainer.

6. An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot is of a crosssectional configuration having a peripheral opening of less transverse dimension than the maximum transverse dimension of the slot, said unit comprising: a flexible retainer embodying a flexible enlargement shaped and dimensioned to be retained in the slot against radial displacement and a flexible web of a thickness enabling it to extend radially outward through the slot, and a plurality of flexible abrasive leaves anchored to the web of the retainer.

7. An abrasive unit as defined in claim 6 in which the retainer comprises a loop of flexible sheet material, a filler encased in an inner portion of the loop, the filler and inner portion of the loop formingthe enlargement, the outer portion of the loop forming the web.

8. An abrasive unit as defined in claim 7 in which the filler is an elongated member of generally circular cross-section.

9. An abrasive unit as defined in claim 7 in which the tiller is a rod.

10. An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot is of a crosssectional configuration having a peripheral opening of less transverse dimension than the maximum transverse dimension of the slot, said unit comprising: a retainer embodying an enlargement shaped and dimensioned to be retained in the slot against radial displacement and a flexible web of a thickness'enabling it to extend radially outward through the slot, and a plurality of flexible abrasive leaves anchored to the web of the retainer.

11. An abrasive unit as defined in claim in which the retainer comprises a loop of flexiblesheet material, a filler encased in an inner portion of theloop, the tiller and inner portion of the loop forming the enlargement, the outer portion of the loop forming the web.

12. An abrasive unit as defined in claim 6 in which I the enlargement and the web are an integral structure displacement and a resiliently flexible web of a thickness enabling the web to extend radially outwardly through the slot and provide a flexural stressing upon the rotation of the rotary structure,

' and a pack formed from a plurality of flexible leaves anchored near one end to the web of the retainer near the outer radial end of the web and extending radially outwardly beyond the outward and of the web.

15. An abrasive unit as set forth in claim 14 wherein the retainer is made from a plastic material.

16. An abrasive unit as set forth in claim 14 wherein the retainer is made from a nylon.

17. An abrasive unit as set forth in claim 14 wherein the enlargement is a unitary structure having a low coefficient of friction relative to the surface of the slot and the web is unitary with the enlargement.

18. An abrasive unit as set'forth inclaim 14 wherein I the retainer is made from a nylon material.

I l l 

2. An abrasive unit as set forth in claim 1 in which the thickness of the web is dimensioned relative to the width of said entrance for a given range of angular movement of the retainer between opposite limit positions determined by the opposite surfaces of the entrance to the slot; and in which the plurality of abrasive leaves is positioned and dimensioned to form a heel for abutment with the periphery of the hub structure before the retainer reaches a limit position thereby to prevent the web from being forced against a side surface of the entrance to the slot.
 3. An abrasive unit as set forth in claim 1 in which the web is resiliently flexible.
 4. An abrasive unit as set forth in claim 1 in which the retainer is made of plastic material having a relatively low coefficient of friction relative to the surfaces of the slot.
 5. An abrasive unit as set forth in claim 1 in which the retainer comprises sheet material encasing a solid core to form the retainer enlargement and in which two layers of the sheet material form the web of the retainer.
 6. An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot is of a cross-sectional configuration having a peripheral opening of less transverse dimension than the maximum transverse dimension of the slot, said unit comprising: a flexible retainer embodying a flexible enlargement shaped and dimensioned to be retained in the slot against radial displacement and a flexible web of a thickness enabling it to extend radially outward through the slot, and a plurality of flexible abrasive leaves anchored to the web of the retainer.
 7. An abrasive unit as defined in claim 6 in which the retainer comprises a loop of flexible sheeT material, a filler encased in an inner portion of the loop, the filler and inner portion of the loop forming the enlargement, the outer portion of the loop forming the web.
 8. An abrasive unit as defined in claim 7 in which the filler is an elongated member of generally circular cross-section.
 9. An abrasive unit as defined in claim 7 in which the filler is a rod.
 10. An abrasive unit for mounting in a peripheral slot of a rotary hub structure wherein the slot is of a cross-sectional configuration having a peripheral opening of less transverse dimension than the maximum transverse dimension of the slot, said unit comprising: a retainer embodying an enlargement shaped and dimensioned to be retained in the slot against radial displacement and a flexible web of a thickness enabling it to extend radially outward through the slot, and a plurality of flexible abrasive leaves anchored to the web of the retainer.
 11. An abrasive unit as defined in claim 10 in which the retainer comprises a loop of flexible sheet material, a filler encased in an inner portion of the loop, the filler and inner portion of the loop forming the enlargement, the outer portion of the loop forming the web.
 12. An abrasive unit as defined in claim 6 in which the enlargement and the web are an integral structure formed of a material to provide the web with flexible characteristics.
 13. An abrasive unit as defined in claim 12 in which the plastic is nylon.
 14. An abrasive unit for mounting in a peripheral slot of a rotary structure wherein the slot is of a cross-sectional configuration having a peripheral opening of less transverse dimension than the maximum transverse dimension of the slot, said unit comprising: a retainer embodying an enlargement shpaed and dimensioned to be retained in the slot against radial displacement and a resiliently flexible web of a thickness enabling the web to extend radially outwardly through the slot and provide a flexural stressing upon the rotation of the rotary structure, and a pack formed from a plurality of flexible leaves anchored near one end to the web of the retainer near the outer radial end of the web and extending radially outwardly beyond the outward end of the web.
 15. An abrasive unit as set forth in claim 14 wherein the retainer is made from a plastic material.
 16. An abrasive unit as set forth in claim 14 wherein the retainer is made from a nylon.
 17. An abrasive unit as set forth in claim 14 wherein the enlargement is a unitary structure having a low coefficient of friction relative to the surface of the slot and the web is unitary with the enlargement.
 18. An abrasive unit as set forth in claim 14 wherein the retainer is made from a nylon material. 